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How to Check for Vowels in Python Strings and Lists

This guide explains how to check if a string contains vowels, how to identify vowels within a string, and how to find words starting with vowels in a list. We'll use efficient and Pythonic techniques, including the in operator, list comprehensions, and generator expressions.

Checking if a String Contains Any Vowels​

The most Pythonic way to check if a string contains at least one vowel is to use the any() function with a generator expression:

vowels = {'a', 'e', 'i', 'o', 'u', 'A', 'E', 'I', 'O', 'U'}  # Use a set for efficiency

my_str = 'tutorialreference'

if any(char in vowels for char in my_str): # Generator expression
    print('The string contains at least one vowel')
else:
    print('The string does NOT contain any vowels')

print(any(char in vowels for char in 'reference'))  # Output: True
print(any(char in vowels for char in 'rfnc'))       # Output: False
print(any(char in vowels for char in ''))           # Output: False
  • A set containing all vowels is created for efficiency.
  • any(char in vowels for char in my_str): This is a generator expression. It efficiently checks each character (char) in my_str to see if it's present in the vowels set. any() returns True as soon as it finds a vowel (short-circuiting), and False if it reaches the end of the string without finding a vowel.
  • Using set for vowels: Using a set ({'a', 'e', ...}) for vowels is significantly more efficient than using a list or string for the in check. Set lookups are O(1) (constant time) on average, while list/string lookups are O(n) (linear time).

Using a for Loop​

You can use a for loop, but it's less concise and less efficient:

vowels = {'a', 'e', 'i', 'o', 'u', 'A', 'E', 'I', 'O', 'U'}

def contains_vowels(string):
    for char in string:
        if char in vowels:
            return True  # Return immediately upon finding a vowel
    return False  # No vowels found after checking all characters
  • Using the for loop to find the vowel is more verbose and less efficient than using any().

Identifying and Extracting Vowels from a String​

To get a list of the vowels present in a string (without duplicates):

vowels = {'a', 'e', 'i', 'o', 'u', 'A', 'E', 'I', 'O', 'U'}
my_str = 'abcde'
vowels_in_string = []

for char in my_str:
    if char in vowels:
        print(f'{char} is a vowel')
        if char not in vowels_in_string: # Check for repetition
            vowels_in_string.append(char)
    else:
        print(f'{char} is a consonant')

# Output:
# a is a vowel
# b is a consonant
# c is a consonant
# d is a consonant
# e is a vowel

print(vowels_in_string)  # Output: ['a', 'e']
  • This uses a for loop to iterate over all the characters, and appends the vowels to a new list, only if not already appended.

Counting Vowels in a String​

To count the number of vowels in a string:

vowels = 'aeiou'  # Only lowercase for simplicity; add 'AEIOU' if needed.
my_str = 'tutorialreference.com'
vowels_count = {vowel: my_str.lower().count(vowel) for vowel in vowels}
print(vowels_count) # Output: {'a': 1, 'e': 4, 'i': 1, 'o': 2, 'u': 1}
  • This uses a dictionary comprehension to create a dictionary that stores the counts of each vowel in the string.

Checking if a Single Character is a Vowel​

To check if a single character is a vowel, the in operator is the most direct and readable way:

vowels = ['a', 'e', 'i', 'o', 'u', 'A', 'E', 'I', 'O', 'U'] # Or use a set
my_str = 'tutorialreference'

if my_str[0] in vowels: # Check if the character is a vowel
    print('The letter is a vowel')
else:
    print('The letter is a consonant') # Output: The letter is a consonant
  • The in operator is used to check if a character is present in a string or a list.

Finding Words Starting with Vowels in a List​

To find words in a list that begin with a vowel:

Using a List Comprehension​

my_list = ['one', 'two', 'age', 'hello', 'example']
vowels = 'aeiouAEIOU'

starting_with_vowel = [word for word in my_list if word[0] in vowels]
print(starting_with_vowel)  # Output: ['one', 'age', 'example']

Using a for Loop​

my_list = ['one', 'two', 'age', 'hello', 'example']

vowels = 'aeiouAEIOU'
starting_with_vowel = []
for word in my_list:
    if word[0] in vowels:
        starting_with_vowel.append(word)    # Append to the list if starts with vowel.
print(starting_with_vowel)                  # Output: ['one', 'age', 'example']

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